(CBS News) Researchers at Northwestern University have found that normal-weight people who develop Type 2 diabetes have a higher mortality rate than overweight or obese people who develop the disease.

Although weight is one of the factors that puts a person at risk of developing Type 2 diabetes, family history, ethnicity and age may also play a role. Since normal-weight people only account for 5 to 15 percent of new diabetes cases, not much is known about how the disease affects this under-studied group.

"It could be that this is a very unique subset of the population who
are at a particularly high risk for mortality and diabetes, and it is
possible that genetics is a factor with these individuals," said
Mercedes R. Carnethon, associate professor of preventive medicine at
Northwestern University Feinberg School of Medicine and first author of
the study, said in the press release.

For the study, which was published in the Aug. 7 issue of JAMA, scientists identified 2,625 men and women in the U.S. from five earlier studies who were over the age of 40 and had developed diabetes. The death rate among normal weight diabetes patients was 284.8 per 10,000 person-years, while the death rate for overweight or obese patients was 152.1 per 10,000 person-years.

After adjusting for demographics, blood pressure, waist circumference, smoking status and lipid levels, the researchers found that normal weight individuals were twice as likely to die from diabetes than overweight/obese people. They also found that older adults and non-white participants were more likely to develop normal-weight Type 2 diabetes.

The researchers cautioned that when testing for diabetes, doctors should not ignore Asians, who typically have a lower body weight than non-Asians, and the elderly, since the average onset age of diabetes is between 65 and 75 years old.

"Many times physicians don't expect that normal-weight people have diabetes when it is quite possible that they do and could be at a high risk of mortality, particularly if they are older adults or members of a minority group," Carnethon said in the press release.

One possible explanation as to why that normal weight people are at a higher risk may be due to fitness. Dr. Hermes Florez, director of the division of epidemiology and population health sciences at the University of Miami's Miller School of Medicine, suggested to WebMD that people who don't maintain a healthy lifestyle might be putting themselves at risk for diseases, even if they are at a normal weight.

"It's not just the issue of fatness. It's also the issue of fitness," he told WebMD.

Florez was not involved with the study, but wrote an accompanying editorial, in which he pointed out that 79 million Americans aged 20 or older have prediabetes and nearly 50 million Americans are expected to have the disease by 2050. Currently, there are 25.8 million people or 8.3 percent of the U.S. population living with diabetes, with an estimated 7.0 million people who are unaware they have the disease, the CDC reports.

Carnethon said in a blog post on JAMA that the study doesn't mean that people with diabetes who are normal weight should gain weight or those who are overweight or obese should avoid losing weight.

"We wanted to communicate to physicians in the clinical setting that relying solely on BMI is problematic because people who may not otherwise raise your clinical suspicions may have clinical problems," she explained.